Only split the signed message in to 2 parts with the message verifier

[jwoertink]

Purpose

Fixes #1595

Description

In rare cases, the generated data can contain the -- sequence in it. If it does, this PR ensures that the string will only ever be split in to 2 parts. The first part is the data, and the second part is the digest.

Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test

[jwoertink]

hmm 🤔 not sure why nightly is failing now... I hope that doesn't mean some sort of breaking change coming up..

[jwoertink]

Thanks for all of the reviews @matthewmcgarvey!